Microscopic voltage controlled field emission cathode-anode structures have been fabricated as individual units and in high density arrays including thousands of devices. Such field emission cathode arrays are constructed in accordance with advanced semiconductor microfabrication technology, including thin film deposition, photolithography, electron lithography, and wet and dry etching processes. Packing densities of 1.2.times.10.sup.6 tips per square centimeter and more have been achieved. Small arrays with the same packing density also have been constructed.
Prior art field emission cathode arrays operate all of the field emission devices in parallel, and the multiple tip arrays have been used for high current density operations. The devices are mounted in a high vacuum housing to avoid disruptions of the emitter cathodes during operation. Thus, field emission cathode array devices comprise miniature vacuum devices. Applications for such devices, however, have been limited; and much work on field emission cathode array devices has been restricted to laboratory experiments.
Fabrication and operating characteristics of known field emission cathode devices utilizing molybdenum cathode cones are described in the technical articles by Charles A. Spindt, et al., in the JOURNAL OF APPLIED PHYSICS, Volume 47, Number 12, December, 1976, Pages 5248 to 5263, and APPLICATIONS OF SURFACE SCIENCE 16 (1983), Pages 268 to 276. The field emission cathode arrays described in those articles essentially comprise of a silicon substrate which has a thermally grown silicon dioxide film on it. A molybdenum anode or gate film is deposited on the surface of the silicon dioxide film. A microscopic array of holes then is micromachined through the anode or gate film and the silicon dioxide layer to the underlying silicon substrate. Molybdenum cones then are formed on the silicon substrate by electron beam evaporation or other suitable technique to produce sharp pointed cone cathodes on the silicon substrate. The tips of the cones are centered in the holes and are located in the plane of the molybdenum anode or gate film. The tips are formed in all of the holes simultaneously by a combination of physical deposition processes, so that the number and packing density of the tips depends only on the number and packing density of the holes which can be formed in the structure. A process or fabricating such devices is described clearly in the abovementioned JOURNAL OF APPLIED PHYSICS article.
The application of a suitable electrical bias between the silicon substrate and the gate film layer, after the array is mounted in a vacuum, causes the emission of electrons from the field emission cathodes. These emitted electrons then are directed to a collector to permit the electron flux thus produced by the array to be used as the electron source for a variety of different electronic devices. The electron flux or current depends strongly upon the bias voltage between the cathode and gate or anode. This current also is dependent upon the sharpness or radius of curvature of the field emission cathode cones. For low voltages, little or no electron current flows, and the current increases sharply with increasing voltage.
Field emission cathode devices have been used as discrete point cathodes as electron sources for scanning electron microscopes. These devices have the advantage of high brightness (electron flux density) and simplicity, since the devices do not require a heating circuit as is required for thermionic cathodes. A significant disadvantage for conventional field emission cathode devices is the extreme sensitivity to residual gas in the vacuum. As a consequence, ultra high vacuum levels have been required, in the range of 10.sup.-9 TORR to prevent ionic bombardment and erosion of the cathode.
An advantage of the microscopic field emission cathode array structures described in the above-identified articles by Spindt et al. is that such ultra high vacuum levels are not required, because the accelerating voltages are small for the microscopic distances involved. In addition, arrays of cathodes including millions of structures are feasible, utilizing the technology described in the Spindt articles.
Accordingly, it is desirable to incorporate the advantages of the high packing densities, relatively low vacuum requirements, and the other inherent advantages of microscopic field emission cathode arrays in configurations which also permit individual control of each of the field emission cathodes of an array independently of the other field emission cathodes in the same array.